Hello @Sriramsubramaniyan Nadarajan , Thanks for using Microsoft Q&A Platform.
It is possible that you have not installed the required package or that there is an issue with your installation.
If you are using latest preview API version, make sure that you have installed the azure-ai-documentintelligence
package.
Install the Azure Document Intelligence client library for .NET with NuGet:
dotnet add package Azure.AI.DocumentIntelligence --prerelease
Here is the GitHub reference related to DocumentIntelligenceClient.cs: https://github.com/Azure/azure-sdk-for-net/blob/Azure.AI.DocumentIntelligence_1.0.0-beta.1/sdk/documentintelligence/Azure.AI.DocumentIntelligence/src/DocumentIntelligenceClient.cs
I hope this helps.
Regards,
Vasavi
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